The mechanical properties of ceramics with the compositions of 5 mol% YO1.5-TZP (ZY5) and 4 mol% YO1.5-4 mol% CeO2-TZP (ZY4Ce4) were investigated. Nanocrystalline powders of both TZPs were synthesized by a gel precipitation technique. Powder compacts were made by cold isostatic pressing, and were then pressureless sintered or sinter-forged to dense TZP ceramics (density: 96–99%) with grain sizes of 180 nm. Shear deformation during sinter-forging was found to favourably affect densification and microstructure. The mechanical properties of both types of ultrafinegrained TZP ceramics were examined up to 760 °C. The results point towards an improved grain boundary structure and the elimination of residual flaws obtained by the sinter-forging technique. The sinter-forged samples exhibited a higher Vicker hardness compared to that of pressureless sintered specimens. After sinter-forging the fracture energy value was found to be 325 J/m2 and the fracture toughness to be 10 MPa m1/2. A Weibull modulus of 21 was obtained for the sinter-forged samples which was significantly higher than the value of 8 for the pressureless sintered specimens. The enhancement of reliability is obviously connected to a decrease in the average size and concentration of flaws caused by inhomogeneous powder particle packing due to the presence of irregular agglomerates.